1. Field of the Invention
The present invention relates to a camera, and more specifically, to a single-lens reflex camera employing as a focusing screen a liquid crystal having a variable light diffusion characteristic.
2. Description of the Prior Art
FIG. 1 cross-sectionally shows an example of a conventional finder optical system for use in a single-lens reflex camera. A taking lens 20 is a lens group comprising a plurality of lens elements, and includes an aperture stop 21 between the lens elements. Light, having passed through the taking lens 20 and been reflected by a main mirror 22, is formed into an image on a focusing screen 11. The image formed on the focusing screen 11 is viewed at a pupil EP through a pentagonal roof prism 3 and an eyepiece 5.
A photometric lens 7 and a photometric apparatus 8 are arranged close to the eyepiece 5. The photometric lens 7, which forms a decentered optical system in the view finder optical system, directs off-axial light of the image formed on the focusing screen 11 to the photometric apparatus 8. The photometric apparatus 8 performs photometry by use of the off-axial light.
The light having passed through the main mirror, 22 which is a semi-transparent mirror, is reflected by a sub mirror 23 and directed to a focus detection apparatus 24. The focus detection apparatus 24 performs focus detection by use of the light reflected by the sub mirror 23. Film, which is arranged on an extension of an optical axis AX of the taking lens 20, is not shown.
A camera where the light reflected by the sub mirror 23 is used not only for focus detection but also for photometry and a camera where photometry is performed by use of a part of axial light reflected by a beam splitter arranged between the focusing screen 11 and the main mirror 22 are also known (e.g. Japanese Published Utility Model Application No. S55-10982). In this case, photometry is performed by use of axial light having passed through the main mirror 22 by a photometric apparatus provided at a bottom of a mirror box (not shown) in which the main mirror 22 is arranged.
The focusing screen 11 is a plastic plate having very small concaves and convexes on its surface. By diffusing light by means of these concaves and convexes, the brightness of the finder view is secured and an output of the photometric apparatus 8 is increased to increase photometric accuracy. For the same reason, photometry is generally performed before the release operation under a condition where the aperture stop 21 is opened.
In actual photographing, however, since the aperture stop 21 is stopped down based on an optimum value for exposure control or on the user's intention, the depth of field of the picture taken differs from the depth of field confirmed in the finder. For this reason, it is impossible to confirm the depth of field before photographing. A camera is known which has a pre-view function with which the depth of field at an aperture condition at the time of actual photographing can be viewed. With this camera, however, the finder view may become dark or indistinct, or if the aperture value is changed due to an interchange of the taking lens, the way in which the view finder image is viewed may change.
To solve this problem, various types of cameras have been proposed where in order to enable the user to confirm the depth of field for actual photographing through view finder while securing sufficient finder view brightness and an output of the photometric apparatus, a focusing screen made of liquid crystal (hereinafter referred to as "liquid crystal focusing screen") is used instead of the concaves and convexes to vary the degree of light diffusion (e.g. Japanese Laid-open Patent Application No. H2-20844, Japanese Laid-open Patent Application No. H2-72324, Japanese Published Patent Application No. S61-60420, Japanese Laid-open Utility Model Application No. S54-120933, Japanese Laid-open Patent Application No. S57-124331, Japanese Laid-open Patent Application No. S57-109923, Japanese Published Patent Application S57-37854 and Japanese Published Utility Model Application No. S55-10982). Hereinafter, the characteristic of liquid crystal to diffuse light will be referred to as "light diffusion characteristic"; its tendency, as "diffusivity"; and the degree of light diffusion by liquid crystal, as "diffusion factor".
However, since the actual diffusion characteristic of liquid crystal largely differs from an ideal, even if the control f-number is not changed, the depth of field decreases due to flare with a decrease in open aperture f-number of the taking lens, and variation in brightness due to the control aperture value is large.